import java.util.Scanner;

class S16_pochygame{
  public static void main(String[] args){
    int l, k, t;
    Scanner scn = new Scanner(System.in);

    l = scn.nextInt();
    k = scn.nextInt();

    t = l / (2 * k);
    if(l == (2 * k ))
      t--;

    System.out.println(k * t);
  }
}